Active steering for marine seismic sources

ABSTRACT

A seismic survey system having a source array ( 11 ) coupled to a deflector device ( 15 ) that controls the position of the source array. A positioning system unit ( 16 ) is mounted on the source array to provide a signal to a controller, informing the controller of the current position of the source array so that the controller can control the position of the deflector device ( 15 ) and the coupled source array. A seismic source ( 14 ) on the source array may be triggered when the source array is at a desired location as measured by the positioning system unit. The deflector device ( 15 ) comprises one or more wings ( 18 ) in a generally vertical or, alternatively, in a generally horizontal arrangement disposed adjacent to a central body ( 19 ). The streamlined central body has connection points that allow the deflector device ( 15 ) to be connected to a tow cable ( 13 ) from the tow vessel ( 12 ) and to the source array ( 11 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to seismic exploration and more specifically, tomarine seismic survey systems.

1. Description of the Related Art

Marine seismic exploration investigates and maps the structure andcharacter of subsurface geological formations underlying a body ofwater. For large survey areas, seismic vessels tow one or more seismicsources and multiple seismic streamer cables through the water. Theseismic sources typically comprise compressed air guns for generatingacoustic pulses in the water. The energy from these pulses propagatesdownwardly into the geological formations and is reflected upwardly fromthe interfaces between subsurface geological formations. The reflectedenergy is sensed with hydrophones attached to the seismic streamers, anddata representing such energy is recorded and processed to provideinformation about the underlying geological features.

Three-dimensional (3-D) seismic surveys of a grid provide moreinformation regarding the subsurface formations than two-dimensionalseismic surveys. 3-D surveys may be conducted with up to twelve or morestreamers that form an array covering a large area behind the vessel.The streamers typically vary in length between three and twelvekilometers. Tail buoys attached at the streamer distal ends carry radarreflectors, navigation equipment, and acoustic transponders. Hydrophonesare positioned along each streamer and are wired together in receivergroups spaced along each streamer. The in-line interval between receivergroups ranges between about 3 and 25 meters, with 12.5 meters comprisingtypical interval spacing. Since the grid is often much wider than thearray, the tow vessel must turn around and tow the array in laps acrossthe grid, being careful not to overlap or leave large gaps between thelaps across the grid.

A multiple streamer array requires diverters near the vessel to pull thestreamers outwardly from the direct path behind the seismic tow vesseland to maintain the traverse or crossline spacing between individualstreamers. Diverters rely on hydrodynamic lift created by forward motionthrough the water to pull the streamers outwardly and to maintain thetransverse position relative to the vessel path. If forward motionchanges due to ocean currents and other environmental factors, thediverters will not maintain the desired streamer position.

In 4-D geophysical imaging, a 3-D seismic survey is repeated over a gridthat has been previously surveyed. This series of surveys taken atdifferent times may show changes to the geophysical image over timecaused, for example, by extraction of oil and gas from a deposit. It isimportant that the sources being used to generate the acoustical pulsesbe located as closely as possible to the same location as in previoussurveys over the same grid. This, of course, has been impossible toaccomplish in a marine survey because the acoustical source arrays aretypically towed behind the tow vessel and are subject to wave andcurrent movement.

In addition to the deployment and operation difficulties associated withtowing multiple streamers, conventional techniques limit the ability toposition source equipment and receivers in different relative positionsand orientations. Because the sources and receivers are towed behind thesame seismic vessel, array design is limited by the tow configuration.Each towed array is also subject to crosscurrents, wind, waves, shallowwater, and navigation obstacles that limit the coverage provided by thesurvey system.

Conventionally, attempts to control the location of source arrays haveincluded attaching the arrays to distance ropes running to otherdeflectors and tow cables. These attempts have not provided optimalcontrol of the location of the source arrays under towing conditions.Furthermore, the distance ropes create a significant drag that must beovercome by the tow vessel and that places a great deal of tension onthe towing cables and attachments.

Accordingly, a need exists for an improved techniques and equipment forconducting marine seismic operations. It would be an advantage if suchtechniques and equipment allowed the acoustic sources to be positionedin desired locations while being towed behind a vessel.

SUMMARY OF THE INVENTION

The present invention provides a seismic survey system for use in waterthat comprises a source array coupled to an independently steerabledeflector device that controls the crossline position of the sourcearray. While the deflector device is preferably connected between thetow cable (or umbilical) and the towed source array, the deflectordevice may also be placed at the distal end of the source array orcontained within the source array.

So that the source array may be tracked as to its present position, apositioning system unit is mounted on the source array. The positioningunit system may be, for example, a global positioning system (GPS) unit,other satellite system, laser, acoustic network or other device thatindicates position. The positioning unit provides a signal to acontroller to inform the controller of the current position of thesource array so that the controller can control the crossline positionof the deflector device and the coupled source array. The controller maybe positioned at a location selected from a towing vessel, thedeflector, and combinations thereof. It should be recognized thatreference to a “controller” encompasses not only a single controller,but also the use of multiple controllers since the distribution ofcontrol among multiple units at one or more locations, and one or morecontrollers controlling a single unit, may still function in the sameway and achieve the same result as a single controller.

The deflector device preferably comprises a single wing in a generallyvertical arrangement and a central body, but may also comprise an upperwing, a lower wing, and a central body, wherein the upper and lowerwings are disposed on opposite sides of the central body in a generallyvertical arrangement and wherein the wings move together in similarmotion. Alternatively, the wings may be arranged horizontally, whereinthe wings may move together in similar motion, or wherein one wing isfixed and the other wing is adjusted, or wherein each wing may beadjusted-independently. An actuator is disposed within or adjacent tothe central body, wherein a controller sends a signal to the actuator,which moves the wing by either electrical or hydraulic action. The oneor more wings have a combined surface area of between about 1 and about7 square meters and may be made of a material selected from metal,plastic composite or combinations thereof. For example, the wing may beconstructed of a metal skin covering a foam core.

Electrical power may be provided to the deflector from the tow vesselthrough the umbilical, from batteries or other electricity storagedevices located on the deflector, and from a combination thereof.

The present invention may further comprise a second source array. Morethan one deflector device may be used, especially when there aremultiple source arrays. Accordingly, the present invention also includesa second independently steerable deflector device coupled to a secondsource array for controlling a crossline position of the second sourcearray.

Additionally, the present invention provides an obstruction avoidancesystem having an acoustical transducer and receiver coupled to thesource array, and a controller, wherein the controller adjusts thedeflector device to steer around an obstruction located by theacoustical transducer and receiver. Preferably, the acousticaltransducer and receiver operate in a range typical for sonar typedevices, and may be used to locate obstructions in or under the watersuch as, for example, installed constructions, moored devices, floatingdevices, lead-in cables and combinations thereof. The acoustictransducer and receiver may be pointed in a given direction oralternatively, the acoustic transducer and receiver may sweep in manydirections. Alternatively, optical devices, such as lasers, may comprisethe obstruction avoidance system.

Also included as the present invention is a method of positioning asource array in tow behind a vessel. The steps of this method includedetermining the crossline position of the source array, andindependently steering a deflector device to position the source arrayin a desired crossline position. The method further includes using acontroller for controlling the deflector device to reach the desiredcrossline position. A positioning system unit is used for determiningthe position of the source array and providing the position to thecontroller.

As previously discussed, the deflector that may be used in the method ofthe present invention preferably comprises a wing and an actuator tomove the wing. The controller controls the actuator, which moves thewing, wherein the movement of the wing steers the source array.

The method of the present invention further provides a method foravoiding obstruction in or under the water. These steps includedetecting acoustic signals or reflections indicating obstructions in thepath of the source array and adjusting the deflector device to steeraround an obstruction detected by the acoustical transducer andreceiver.

The present invention further provides a system for changing theposition of a source array towed by a vessel in a body of water having adeflector coupled to the source array. The deflector includes a wingthat provides a lateral force to the source array as the source array istowed through the water, an actuator for controllably varying the anglebetween the deflector wing and the direction of tow of the vessel, and asensor for measuring the wing angle. Additionally the deflector mayinclude a positioning system for indicating the position of thedeflector, and a controller for providing a command to the actuator forvarying the angle between the deflector wing and the tow direction.Preferably, commands from the controller to the actuator cause thedeflector to steer to a desired position and then maintain thatposition.

The deflector is stabilized against forces transverse to the directionof tow by drag forces resulting from towing the source array from thedeflector. Further stabilization may be realized by having a lower endof the deflector that is weighted and an upper end that is buoyant.

As an alternative seismic survey system for use in water, the presentinvention provides a winch for controlling the position of a sourcearray. The system comprises a source array towed by a first tow cable, adeflector towed by a second tow cable, a distance rope coupling thefirst and second tow cables, and a winch attached to the distance rope,wherein the winch controls a position of the source array. A positioningsystem receiver unit mounted on the source array and a controller areused for controlling the position of the source array. The controllermay be a computer, a distributed control system, an analogue controlsystem or other controller known to those having ordinary skill in theart. The controller is positioned at a location selected from a towingvessel, in-sea (such as at the winch), and combinations thereof. Thepositioning system receiver unit attached to the source array provides asignal to inform the controller of the current location of the sourcearray. A seismic source on the source array may be triggered when thesource array is at a desired location.

The winch comprises any apparatus for varying the effective length ofthe rope. For example, the winch may include a reel for winding thedistance rope onto the winch, an actuator for rotating the reel, and ahousing adjacent to the reel. The actuator uses a motive force selectedfrom electrical and hydraulic. The housing protects sensors, conductors,power sources and power storage units from the water and comprises aconnector for connecting an umbilical containing conductors or fibreoptics to the housing from the controller and power source on the towvessel. Power may be supplied from the tow vessel to the winch throughthe umbilical/tow cable, from one or more batteries located on thewinch, or from combinations thereof.

A source array comprises one or more sub-arrays. Source arrays that aresteered to locations on only one side of the centreline of the towvessel require only one deflector device to control the crosslineposition of the source array because the forces exerted upon the sourcearray as the array is being towed through the water tend to force thesource array to the centre line of the tow boat. Alternatively, if thesource array will be steered to locations on both sides of thecentreline of the tow vessel, then two deflector devices, one on eachside of the source array, are required.

The system may further comprise an obstacle avoidance system comprisingan acoustical transducer and receiver coupled to the source array, and acontroller, wherein the controller adjusts the winch to steer around anobstruction located by the acoustical transducer and receiver. Theacoustical transducer and receiver preferably operate in a range typicalfor sonar type devices.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings wherein like reference numbers representlike parts of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is side view of a vessel towing a source array deployed with adeflector device in accordance with the present invention.

FIGS. 2A-2B are perspective views of each side of the deflector device.

FIG. 3 is a side view of the deflector device coupled to the tow cablebetween the tow vessel and a sub-array in accordance with the presentinvention.

FIG. 4 is a schematic diagram of a control scheme to steer a sourcearray in accordance with the present invention.

FIG. 5 is an aerial view of a marine seismic survey system with multiplesource arrays deployed with distance ropes that couple adjacentsub-source arrays together, being steered with one deflector device oneach source array in accordance with the present invention.

FIG. 6 is an aerial view of a marine seismic survey system with multiplesub-arrays coupled together with distance ropes and each source arraybeing steered with winches in accordance with the present invention.

FIG. 7 is a top view of a winch used to steer a source array inaccordance with the present invention.

FIG. 8 is an aerial view of a tow vessel towing a single source array.

DETAILED DESCRIPTION

The present invention provides a seismic survey system that includes oneor more source arrays towed behind a tow vessel. The system may be usedfor conducting seismic surveys of the subsurface geological formationsthat underlie a body of water. The system comprises a source array andan independently steerable deflector device that is coupled to thesource array, preferably suspended from a float element of the sourcearray or alternatively, from a separate independent float. The deflectordevice controls the position of the source array by exerting a lateralforce against the source array as the deflector device is towed throughthe water. The deflector device preferably includes one wing that may beangularly adjusted to steer the source array to a desired positionbehind the tow vessel.

The deflector device is a hydrodynamic body that uses the watervelocity, achieved by being towed by the tow vessel through the water,to generate a lateral force to steer the source array to the desiredlocation. It should be noted that a towed source array generally movesalong the centreline of the tow vessel due to the forces exerted on thesource array by the water. Therefore, the defector device is usedgenerally to steer the source array away from the centreline of the towvessel to a desired position. The deflector device preferably comprisesa single wing in a generally vertical arrangement and a central body,but may also comprise an upper wing, a lower wing, and a central body,wherein the upper and lower wings are disposed on opposite sides of thecentral body in a generally vertical arrangement and wherein the wingsmove together in similar motion. Alternatively, the wings may bearranged in a generally horizontal arrangement, wherein wings may movetogether in similar motion, or wherein one wing is fixed and the otherwing is adjusted, or wherein each wing may be adjusted independently.The preferably streamlined central body has connection points that allowthe deflector device to be connected to a tow cable from the tow vesseland to the source array. The deflector device may be positioned at thefront of a source array, at the end of a source array, within the sourcearray itself, or there may be multiple deflector devices positioned insome combination of these positions. A preferred location is at thefront of the sub-array located furthest from the centreline of the towvessel so that the source array trails the deflector device.

As the deflector device is towed through the water, the one or morewings produce a sideways force, or “lift”, which moves the deflectorlaterally relative to the direction of tow. It is useful to define an“angle of attack” when discussing such lift, this angle being defined bythe arc between the plane in which the trailing surface of the wing liesand the direction of tow through the water. The angle of attack will liegenerally in a horizontal plane, although not necessarily so.

The central body preferably also contains the actuator that moves thewings as well as connections for an umbilical, or cables, hoses andcombinations thereof, that carry control signals to and from thedeflector device. The actuator may move the wings hydraulically or by anelectric motor. An electric motor is preferred because an electric motoris smaller, simpler and less expensive to operate and maintain than ahydraulic system. The central body may also contain sensors that sensethe motion and position of the wing and transmit that information to acontroller discussed below. Alternatively, the actuator may be adjacentto the central body.

Electrical power may be supplied to the deflector through conductors inthe umbilical, or cable, from the tow vessel, from batteries or otherelectricity storage devices located on the deflector, or fromcombinations thereof.

The wing or hydrofoil may have a profile that is symmetrical or that hascamber. A profile with camber is preferred. The size of the wing, oralternatively the wings, will normally be in the range of between aboutone and seven square meters total for each deflector. The wing may beconstructed of any material, but preferably the wing is made from metal,composite materials or combinations thereof. One preferred constructionis a stainless steel skin covering a foam core. If weight is a concern,titanium may be used as a preferred material of construction. Thecentral body and actuator are preferably constructed of stainless steel.The submerged weight of the deflector device is preferably between about30 and about 150 kg. Foam filling the wings and central body may be usedfor buoyancy to counteract the weight of the metal used for constructingthe deflector device.

A control system monitors the location of the source array and sendssignals to the deflector device to steer the array to the desiredlocation for the array. While the deflector may be used for positioninga source array relative to the vessel, it is preferably used forpositioning the source array relative to the globe. Therefore, apositioning system unit is attached to the source array to provide thecontroller with the actual location of the source array. By knowing theexact location of the positioning system unit and knowing the in-linelocation of each acoustic source in the array relative to thepositioning system unit, the controller can roughly determine thelocation of each acoustic source being towed behind the tow vessel. Thepositioning system unit may be, for example, a global positioning system(GPS) unit, other satellite positioning system, laser, an acousticnetwork, or any other type of unit known to one having ordinary skill inthe art that may be used to determine a specific location.

The controller may compare the actual location of the positioning systemunit with the desired location and then send a signal to the deflectordevice to steer the source array and thereby maintain or achieve thedesired location. Since the controller is constantly monitoring thelocation of the source array, if the array is moved due to currents,waves, tides, winds or other outside forces, the controller may quicklysense the movement and instruct the deflector device to steer the sourcearray back to the desired location. The controller may be located on thetow vessel communicating with the actuator that moves the wings on thedeflector device. In a preferred embodiment, an on-board controller onthe tow vessel communicates with a local controller in the central bodyof the deflector device. This local controller then transmits a signalto the actuator to move the wing and steer the source array to theoptimum desired location. In this embodiment, the positioning systemunit sends a signal to the navigation system of the tow vessel, whichthen communicates the location of the source array to the on-boardcontroller. Using the navigation system of the tow vessel to receive thepositioning system unit's signal is a preferred embodiment because thenavigation system of the tow vessel conventionally has the capability ofreceiving and processing the signal from the positioning system unit.Alternatively, the capability of receiving and processing the signalfrom the positioning system unit may be made part of the on-boardcontroller. Sensors in the central body may also monitor the position ofthe wing and/or the movement of the deflector device and transmitsignals providing that information to the local controller and theon-board controller, if desired.

The on-board controller may be a computer, a distributed control system,an analogue control system or other control system known to those havingordinary skill in the art. The local controller may be one or moredigital controllers, analogue controllers or combinations thereof. Theon-board controller and the local controller may send control signalsand receive transmitter signals or signals from each other by any means,including radio waves, electrical conductors, fibre optics orcombinations thereof.

The seismic survey system may also include an obstructions avoidancesystem. An acoustical transducer and receiver may be mounted on thedeflector device or on an adjacent source array. The acousticaltransducer and receiver preferably operate in the range of typical sonarsystems and may be directed in either one general direction or sweep inmany directions. The acoustical transducer and receiver may be used tolocate obstructions in or under the water such as, for example, underseaconstructions, moored devices, free floating devices, tow cables andtowed devices. When the acoustical transducer and receiver locates anobstruction, a signal may be sent to the controller (either the on-boardcontroller or the local controller) and the controller may then signalthe deflector device to adjust the wing and steer the source array awayfrom or around the sensed obstruction. The acoustical transducer andreceiver may be powered from the local in-sea electronics. Communicationsignals from the acoustical transducer and receiver to the controllermay be transmitted by any means, including radio waves, optical fibresor electrical conductors, preferably by conductors in the umbilicalattached to the deflector device. This obstructions avoidance system maybe particularly useful during recovery or deployment of the array toavoid tangling of the tow cables.

If a tow vessel is towing more than one source array, each individualsource array typically has one deflector device to control each sourcearray's location. When the source array comprises two or more sub-arrayscoupled together with distance ropes, only one of the coupledsub-arrays, typically the sub-array located furthest from the towvessel's centreline, is provided with a deflector device to steer thesource array.

When a tow vessel is towing only one source array comprising a pluralityof sub-arrays, then both the sub-array furthest and the sub-arrayclosest to the centreline of the tow vessel are provided with adeflector device to steer and maintain the source array to desiredlocations on either side of the tow vessel's centreline.

Alternatively, a winch may be used, instead of the deflector device, tocontrol the location of a source array. Using a monowing or similardevice to control the location of seismic steamers, an adjustabledistance rope coupling a lead-in of a sub-array of the source array tothe lead-in of a seismic streamer can control the position of the sourcearray. The distance rope length is adjusted by means of a winch that isattached to the lead-in of the sub-array farthest from the centreline ofthe tow vessel. The adjustable distance rope that positions the sourcearray may be shortened or lengthened by the winch, effectively steeringthe source array to a desired position. A controller may control thewinch and thereby lengthen or shorten the length of the distance rope,winching the rope in or out to steer the source array to the desiredposition. As in the embodiment with the deflector device, a positioningsystem unit may be mounted on the source array to measure the actuallocation of the source array and to provide the controller with thatinformation. However, instead of adjusting the position of the wing onthe deflector device, the controller may adjust the effective length ofthe distance rope by winching the distance rope in or out. The distancerope may be made of rope, chain, wire or other form of material that maybe winched and that is suitable for the sea environment as known tothose having ordinary skill in the art.

It is an important advantage of the present invention that the deflectordevice, or alternatively the winch, steers the source array to apredetermined position and maintains that position while the array isbeing towed through the water. The predetermined position may be along astraight line or along any other track that has been defined either fromexperience or from previous surveys, or the position may be one thatwill simply enable optimum source positions in future surveys.Furthermore, during 4-D seismic surveys, it is important that the sourcearray be located as closely as possible to the same locations usedduring previous surveys of the same grid. It is an advantage of thepresent invention that the deflector may be used to locate the sourcearray at the same location as used during previous surveys of the grid.

Without the deflector device or winch steering the source array, theposition of the source array is subject to the influence of currents,waves, wind and changes in the direction of the tow vessel. By exertinglateral force on the water, the deflector device or winch can steer thesource array to the optimum predetermined position independent of thelocation of streamers, the speed of the tow vessel or other influences.

Another advantage of the present invention is that the increased controlover the location of the source array allows the source array to bepositioned closer to installed constructions, moored units or otherknown devices in or under the water without fear of entangling the towcables or arrays with these obstacles. This provides increased seismicsurveying of a grid by being able to gather seismic data from locationsthat before were avoided for fear of entangling the seismic surveyequipment with obstacles in or under the water. Furthermore, thecross-line positioning control may be used to decrease the turningradius of the streamer vessel without tangling the streamers.

FIG. 1 is a side view of a vessel towing a source array deployed with adeflector device in accordance with the present invention. A tow vessel12 tows a sub-array 11 of a source array by a tow cable 13. Thesub-array 11 has several acoustic sources 14 suspended from thesub-array 11. These acoustic sources 14 are traditionally compressed airguns, which are fired to generate acoustical waves that are reflectedfrom the subsurface geological features back to receivers (not shown)during a seismic exploration. The deflector device 15 is connected tothe tow cable 13 between the vessel 12 and the source array 11 such thatthe source array 11 trails the deflector device 15. Also mounted on thesource array 11 is a global positioning system (GPS) unit 16 thatnotifies the navigation system 17 of the tow vessel 12 of the exactlocation of the source array 11.

FIGS. 2A-2B are perspective views of each side of the deflector device15. The moveable wing 18 is disposed adjacent to the streamlined centralbody 19. The actuator 20 moves the wing 18 about the wing's verticalaxis. The central body 19 may also contain a local controller 24 andsensors (not shown) for monitoring the movement of the wing 18 andcontain the motor (not shown) that drives the actuator 18 andoptionally, batteries 23. The tow cable 13 attaches to the deflector fortowing the deflector by the tow vessel and connects the deflector 15 tothe sub-array 11 being towed. Electrical, optical or a combination ofcables 22 thereof may be connected to the deflector to provideelectrical power and to carry control signals. A high pressure hose 21may also be connected to the deflector to provide high pressure air tothe air guns.

FIG. 3 is a side view of the deflector device coupled to the tow cablebetween the tow vessel and a sub-array in accordance with the presentinvention. The tow cable 13 tows the sub-assembly 11 and the deflectordevice 15 though the water. The central body 19 is adapted to beconnected to the tow cable 13 between the tow vessel and the sub array15. The electrical, optical, or combination of cables 22 may containelectrical power conductors, control signal conductors and fibre opticsfor sending and/or receiving electrical power and control signals and isshown connected to the central body 19. Also shown connected to thecentral body 19 is a high pressure hose 21 for supplying high pressureair to the air guns. The bulkhead 27 provides connections for the cable22 and high pressure hose 21. Ropes 26 connected to the sub-array float25 support the deflector device 15. Positioning units, GPS unit 16 andacoustic sensor 23 are shown mounted to the float 25 and acousticsources 14 are shown suspended from the float 25.

FIG. 4 is a schematic diagram of a control scheme to steer a sourcearray in accordance with the present invention. The positioning unit 16,17 mounted on the sub-array 11 (FIG. 1) transmits the position of thesub-array 11 to the navigation system 17 located on the tow vessel 12(FIG. 1). The navigation system 17 provides the location informationreceived from the positioning unit 16 to the on-board controller 32. Theon-board controller 32 may be a computer, a distributed control system,an analogue control system or other control device known to those havingordinary skill in the art. The on-board controller 32 preferablycommunicates with the local controller 29 through the umbilical 27, butmay communicate through a wireless transmission. The umbilical 27contains conductors for providing power and control signals to and fromthe central body 19. The local controller 29 sends a signal to theelectric motor 31 that moves the actuator 20, which in turn moves thewing 18. When the wing 18 moves, the lateral force imparted against thewing by the water steers the sub-array 11 to the desired position.Sensors 28 detect the angular position of the wing 18 and send thisinformation back to the local controller 29 and, optionally, to theon-board controller 32 where it may be displayed for an operator toread.

FIG. 5 is an aerial view of a marine seismic survey system with multiplesource arrays deployed with distance ropes that couple adjacentsub-source arrays together, being steered with one deflector device oneach of the source arrays in accordance with the present invention.Multiple sub-arrays 11 are shown forming two source arrays deployed withdistance ropes 33 that couple adjacent sub-arrays 11 together. Each ofthe source arrays are shown being steered with one deflector device 15attached to the sub-array 11 furthest from the centreline of the towvessel 12. The distance ropes 33 maintain a set distance between each ofthe adjacent sub-arrays 11 coupled together with the distance ropes 33or may optionally include a winch as previously described. One deflectordevice 15 may then be used to steer the coupled sub-arrays 11 as oneunit to a desired position.

FIG. 6 is an aerial view of multiple sub-arrays 11 coupled together withdistance ropes 33, with each source array being steered with a winch 36in accordance with the present invention. In this embodiment, a distancerope 33 maintains the relative distance between a deflector 37 and thesource arrays 38. The deflector 37 may be dedicated to the source arraysas shown, or alternatively, the deflector 37 may be connected with aseismic streamer cable that is connected to the source arrays with adistance rope as shown in FIG. 5. A winch 36 may adjust the length ofthe distance rope 33 by winching the distance rope 33 in or out. As thewinch 36 winches the distance rope 33 in, the source array 38 movestowards the deflector 37. As the winch 36 winches the distance rope 33out, the source array 38 moves away from the deflector 37. The on-boardcontroller 32 receives a signal transmitted by the GPS unit 16 mountedon one of the sub-arrays 11. The on-board controller 32 then transmits acontrol signal to the winch 36 to winch the distance rope 33 in or out,thereby steering the source arrays 11 to a desired position ormaintaining a desired position.

FIG. 7 is a top view of a winch used to steer a source array inaccordance with the present invention. The winch 36 is attached to thetow cable 13 between the tow vessel 12 and the sub-array 11 as shown inFIG. 6. The reel 40 rotates to take up or let out the distance rope 33.The reel 40 may be turned with an electric or hydraulic motor 39 poweredfrom the cable 22. (FIG. 3) Batteries 35, or other energy storage unit,may also be included to provide power. A controller unit 41 may beprovided for controlling the winch motor 39.

FIG. 8 is an aerial view of a tow vessel towing a single source array.The tow vessel 12 is towing a single source array 38 comprising threesub-arrays. Because of the forces exerted upon the source array 38 whileit is being towed in the water, the source array 38 tends to follow thecentreline of the tow vessel 12. To steer the source array 38 to eitherside of the centreline, two deflector devices must be provided, one ofeach of the sub-arrays 11 furthest from the centreline of the tow vessel12. Alternatively, a winch 36 (FIG. 7) may replace the deflector devices15. Alternatively, if the source array 38 is to be steered to only oneside of the centreline of the tow vessel 12, then only one deflectordevice 15 would be required.

It will be understood from the foregoing description that variousmodifications and changes may be made in the preferred embodiment of thepresent invention without departing from its true spirit. It is intendedthat this description is for purposes of illustration only and shouldnot be construed in a limiting sense. The scope of this invention shouldbe limited only by the language of the following claims.

1. A seismic survey system for use in water, comprising: a source array;an independently steerable deflector device coupled to the source array,wherein the deflector device controls a position of the source array;and a positioning system to determine the location of the source array.2. The seismic survey system of claim 1, wherein the deflector devicecontrols the position of the source array that trails the independentlysteerable deflector device.
 3. The seismic survey system of claim 1,wherein the deflector device controls the position of the source arraythat is coupled to a front of the independently steerable deflectordevice.
 4. The seismic survey system of claim 1, wherein the deflectordevice controls the position of the source array containing theindependently steerable deflector device.
 5. The seismic survey systemof claim 1, further comprising: wherein the positioning system comprisesa positioning unit mounted on the source array.
 6. The seismic surveysystem of claim 5, wherein the positioning system is selected from aglobal positioning system, an acoustic network, and a laser system. 7.The seismic survey system of claim 5, wherein the positioning system isa satellite positioning system.
 8. The seismic survey system of claim 1,further comprising: a controller for controlling the position of thedeflector device.
 9. The seismic survey system of claim 8, wherein thedesired position is the same position as in a previous seismic survey.10. The seismic survey system of claim 8, wherein the desired positionis a set distance from an edge of a previous seismic survey.
 11. Theseismic survey system of claim 10, wherein the desired position avoidsgaps in coverage.
 12. The seismic survey system of claim 8, furthercomprising: a positioning unit attached to the source array, wherein thepositioning unit provides a signal to inform the controller of a currentposition of the source array.
 13. The seismic survey system of claim 12,wherein a seismic source on the source array is triggered when thesource array is at a desired position.
 14. The seismic survey system ofclaim 8, wherein the controller is positioned at a location selectedfrom a towing vessel, the deflector device, and combinations thereof.15. The seismic survey system of claim 1, wherein the deflector devicecomprises: one or more wings; and a central body, wherein the one ormore wings are disposed adjacent to the central body.
 16. The seismicsurvey system of claim 15, wherein the one or more wings are in agenerally vertical arrangement.
 17. The seismic survey system of claim15, wherein the one or more wings are in a generally horizontalarrangement.
 18. The seismic survey system of claim 15, furthercomprising: an actuator disposed adjacent the central body, wherein acontroller sends a signal to the actuator, and wherein the actuatormoves the one or more wings.
 19. The seismic survey system of claim 18,wherein the actuator uses a motive force selected from electrical andhydraulic.
 20. The seismic survey system of claim 18, wherein thecentral body and the actuator are made of a material selected frommetal, composite and combinations thereof.
 21. The seismic survey systemof claim 15, wherein the total area of the one or more wings is betweenabout 1 and about 7 square meters.
 22. The seismic survey system ofclaim 15, wherein the upper and lower wings are constructed of amaterial selected from metal, composite or combinations thereof.
 23. Theseismic survey system of claim 15, wherein the one or more wings areconstructed of a metal skin covering a foam core.
 24. The seismic surveysystem of claim 23, wherein the metal skin is selected from titanium andstainless steel.
 25. The seismic survey system of claim 1, wherein thesource array comprises one or more sub-arrays and wherein the sub-arraysare coupled to adjacent sub-arrays within the source array by a distancerope.
 26. The seismic survey system of claim 1, further comprising: asecond independently steerable deflector device coupled to a secondsource array for controlling a second position of the second sourcearray.
 27. The seismic survey system of claim 1, further comprising: anacoustical transducer and receiver coupled to the source array; and acontroller, wherein the controller adjusts the deflector device to steerclear of an obstruction located by the acoustical transducer andreceiver.
 28. The seismic survey system of claim 27, wherein theacoustical transducer and receiver operate in a range typical for sonardevices.
 29. The seismic survey system of claim 27, wherein theobstruction is selected from the group consisting of installedconstructions, moored devices, floating devices, lead-in cables,umbilicals, towed equipment and combinations thereof.
 30. The seismicsurvey system of claim 27, wherein the acoustic transducer and receiverare pointed in a given direction.
 31. The seismic survey system of claim27, wherein the acoustic transducer and receiver sweeps in manydirections.
 32. A method of positioning a source array in tow behind avessel, comprising: determining the position of the source array; andindependently steering a deflector device coupled to the source array tomove the source array to a desired position.
 33. The method of claim 32,wherein the deflector device is coupled to a front end of the sourcearray.
 34. The method of claim 32, wherein the deflector device iscoupled to a back end of the source array.
 35. The method of claim 32,wherein the deflector device is coupled within the source array.
 36. Themethod of claim 32, wherein the step of determining the position furthercomprises: determining the position of the source array.
 37. The methodof claim 32, further comprising: controlling the deflector device tosteer the source array to the desired position.
 38. The method of claim37, wherein the desired position is a same position as in a previousseismic survey.
 39. The method of claim 37, wherein the desired positionis a set distance from an edge of a previous seismic survey.
 40. Themethod of claim 39, wherein the desired position avoids gaps incoverage.
 41. The method of claim 37, further comprising: determiningthe position of the source array, providing the position to thecontroller.
 42. The method of claim 41, further comprising: triggering aseismic source on the source array when the source array is at a desiredposition.
 43. The method of claim 37, wherein the deflector devicecomprises: one or more wings; a central body; and an actuator disposedwithin the central body, wherein the one or more wings are disposedadjacent to the central body.
 44. The method of claim 43, furthercomprising: transmitting a control signal to the actuator; moving theone or more wings with the actuator, wherein the movement of the one ormore wings steers the source array.
 45. The method of claim 44, whereinthe actuator uses a motive force selected from electrical and hydraulic.46. The method of claim 43, wherein the central body and the actuatorare made of stainless steel.
 47. The method of claim 43, wherein thetotal surface area of the one or more wings is between about 1 and about7 square meters.
 48. The method of claim 43, wherein the one or morewings are constructed of a material selected from metal, composite orcombinations thereof.
 49. The method of claim 43 wherein the one or morewings are constructed of a metal skin covering a foam core.
 50. Themethod of claim 49, wherein the metal skin is selected from titanium andstainless steel.
 51. The method of claim 32, wherein the source arraycomprises one or more sub-arrays, the method further comprises: couplingthe sub-arrays to adjacent sub-arrays within the source array withdistance ropes.
 52. The method of claim 32, further comprising: couplinga second independently steerable deflector device to a second sourcearray for controlling a second position of the second source array. 53.The method of claim 32, further comprising: detecting acoustic signalsindicating obstructions in the path of the source array; and adjustingthe deflector device to steer clear of an obstruction detected by theacoustical transducer and receiver.
 54. The method of claim 53, furthercomprising: operating an acoustical transducer and receiver in a rangetypical for sonar devices.
 55. The method of claim 53, wherein theobstruction is selected from the group consisting of installedconstructions, moored devices, floating devices, lead-in cables andcombinations thereof.
 56. The method of claim 55, further comprising:pointing the acoustic transducer and receiver in a given direction. 57.The method of claim 53, further comprising: sweeping the acoustictransducer and receiver in many directions.
 58. A system for changingthe position of a source array towed by a vessel in a body of watercomprising: a deflector coupled to the source array, wherein thedeflector includes a wing that provides a lateral force to the sourcearray as the source array is towed through the water; an actuator forcontrollably varying the angle between the deflector wing and thedirection of water flow; a sensor for indicating the position of thesource array; and a controller for providing a command to the actuatorto vary the angle of attack of the deflector body.
 59. The system ofclaim 58, wherein the command from the controller to the actuator causesthe deflector to steer to a desired position.
 60. The system of claim58, wherein the deflector is stabilized against forces transverse to thedirection of tow by drag forces resulting from towing the source arrayfrom the deflector.
 61. The system of claim 60, wherein the deflector isfurther stabilized against forces transverse to the direction of tow bythe deflector having a lower end that is weighted and an upper end thatis buoyant.
 61. The system of claim 58, wherein the sensor is asatellite positioning system sensor.
 62. The system of claim 58, whereinthe actuator is selected from a hydraulic actuator, an electrical motor,and combinations thereof.
 63. A seismic survey system for use in water,comprising: a source array towed by a first tow cable; a deflectordeflecting a second tow cable; a distance rope coupling the first towcable to the second tow cable; and a winch attached to the distancerope, wherein the winch adjusts a length of the distance rope to modifya position of the source array.
 64. The system of claim 58, wherein thedeflector is not supported by a float supporting the source array. 64.The seismic survey system of claim 63, further comprising: a positioningsystem unit mounted on the source array.
 65. The seismic survey systemof claim 63, further comprising: a controller for controlling theposition of the source array.
 66. The seismic survey system of claim 65,wherein the position is the same position as in a previous seismicsurvey.
 67. The seismic survey system of claim 65, wherein the positionis a set distance from an edge of a previous seismic survey.
 68. Theseismic survey system of claim 67, wherein the position avoids gaps incoverage.
 69. The seismic survey system of claim 65, further comprising:a positioning system unit attached to the source array, wherein thepositioning system unit provides a signal to inform the controller ofthe position of the source array.
 70. The seismic survey system of claim69, wherein a seismic source on the source array is triggered when thesource array is at a desired location.
 71. The seismic survey system ofclaim 66, wherein the controller is positioned at a location selectedfrom a towing vessel, the winch, and combinations thereof.
 72. Theseismic survey system of claim 63, wherein the winch comprises: a reelfor winding the distance rope onto the winch; an actuator for rotatingthe reel; and a housing adjacent to the reel.
 73. The seismic surveysystem of claim 72, wherein the actuator uses a motive force selectedfrom electrical and hydraulic.
 74. The seismic survey system of claim72, wherein the housing and the actuator are made of stainless steel.75. The seismic survey system of claim 63, further comprising: anacoustical transducer and receiver coupled to the source array; and acontroller, wherein the controller adjusts the winch to steer clear ofan obstruction located by the acoustical transducer and receiver. 76.The seismic survey system of claim 75, wherein the acoustical transducerand receiver operate in a range typical of sonar equipment.
 77. Theseismic survey system of claim 75, wherein the obstruction is selectedfrom the group consisting of installed constructions, moored devices,floating devices, lead-in cables and combinations thereof.
 78. Theseismic survey system of claim 75, wherein the acoustic transducer andreceiver are pointed in a given direction.
 79. The seismic survey systemof claim 75, wherein the acoustic transducer and receiver sweeps in manydirections.